>>43813
On that map those black lines are some of the possible places where a defense can be mounted relatively easily. He mentions some more. All follows a river or a mountain range.

In comparison here's what I can put together based on his vague description in relation to the German crisis zone.
He refers to it as a pocket which is an encircled area, in order to trap, contain, and crush a military force inside. The encirclement is done by the landing units, while at the Main a backstop for their advance is created by armies joining at the Upper Danube region. The crossing of the Alps is helped by the attack from France, I'm guessing by forcing the defending German units in the Alps out of their position by threatening those by cutting their supply lines, and way of retreat, so basically with the danger of encirclement.
Since he mentions operations in Scandinavia and the Balkans, I assume depending on their advance they could also join in the battle, coming from Denmark and Austria, but those at some point have to cross major rivers, Elbe and the Danube. From the Balkans they could go through Czechia, despite the mountainous areas can be well defended (also rivers in a way again).
Viewing the landscape purely from the landing forces, it's relatively easy. Instead of crossing major rivers, they only need to follow along them. As they would move into the German Uplands, the advance bound to become harder, but still easier than crossing the Rhein from the West.